Completely at the discretion of the counter/gate agents and obviously the Captain. Your jumpseat is only valid for space available in "Y".....if you're fortunate to get a "J" seat sometime from check-in, to closing the L1 door, then someone was going above and beyond to offer you a "professional curtesy"...good luck. Been J/S-ing with AC for almost 7 years now, and never had a hitch yet...